Color printer calibration architecture

ABSTRACT

In a printer which produces images as a function of the combination of cyan (C), magenta (M), yellow (Y) and black (K) colorants on an output color print, responsive to device independent colorimetric description of an image, there is provided a method of calibrating the response of the printer to an image described in terms of colorimetric values, including the steps of first, gray balancing or linearizing colorant signals; secondly, adding black to an ideal device dependent description of a color image in accordance with a predetermined black addition process, and thirdly, providing a color correction transformation process, where the color correction transformation is accomplished via a method of interpolating printer responses from a look-up table indexing colorimetric descriptions of measured real responses, which take into account subsequent black addition and signal linearization.

The present invention is directed to a printer calibration system forcalibrating a printer to produce an accurate printer response based on agiven ideal input image, and more particularly to a calibrationarchitecture which allows device independent color image descriptionconversion to device dependent descriptions that produce an accurateprinter response.

INCORPORATION BY REFERENCE

The following patents are specifically incorporated by reference: U.S.Pat. No. 4,500,919 to Schreiber for its teachings of a color conversionsystem converting information from RGB to CMYK; U.S. Pat. No. 4,275,413to Sakamoto for its teachings of tetrahedral interpolation between firstand second color spaces; and U.S. Pat. No. 2,790,844 to Neugebauerdisclosing the desirability of defining an image in a first standardcolor space prior to conversion of the image coordinates to a secondprinter based coordinate system. The following articles are also herebyincorporated by reference: Po-Chieh Hung, "Tetrahedral DivisionTechnique Applied to Colorimetric Calibration for Imaging Media", AnnualMeeting IS&T, N.J., May, 1992, pp. 419-422; Po-Chieh Hung, "ColorimetricCalibration for Scanners and Media", SPIE, Vol. 1448, Camera and InputScanner System, (1991); and Sigfredo I. Nin, et al., "Printing CIELABImages on a CMYK Printer Using Tri-Linear Interpolation", SPIEProceedings, Vol. 1670, 1992, pp. 316-324.

BACKGROUND OF THE INVENTION

The generation of color documents can be thought of as a two stepprocess: first, the generation of the image by means of scanning anoriginal document with a color image input terminal or scanner or,alternatively, creating a color image on a work station operated inaccordance with a color image creation program; and secondly, printingof that image with a color printer in accordance with the colors definedby the scanner or computer generated image. Scanners typically operatewith colors defined in a color space of tristimulus values, i.e., RGB(red-green-blue). Commonly, these values are a linear transformation ofthe standard XYZ coordinates of CIE color space, or another transform ofthose values.

Printers have an output which can be defined as existing in a colorspace called CMYK (cyan, magenta, yellow, key or black) which isuniquely defined for the printer by its capabilities and colorants.Printers operate by the addition of multiple layers of ink or colorantin layers or halftone dots to a page. The response of the printer tendsto be non-linear. Thus, while a printer receives information in a firstcolor space which has values defined independently of any device, itmust convert that information to print in a second color space which isdependent of device characteristics.

The desirability of operating in a tristimulus color space withsubsequent conversion to a printer colorant color space is well known,as shown by U.S. Pat. No. 4,500,919 to Schreiber, U.S. Pat. No.2,790,844 to Neugebauer, and U.S. Pat. No. 4,275,413 to Sakamoto. Thereare many methods of conversion between color spaces, all of which beginwith the measurement of printer response to certain input values.Commonly, a printer is driven with a set of color input values, thevalues are printed in normal operation of the printer, and measurementsare made of those colors to determine what the actual color printed wasin response to the color specification. As previously noted, mostprinters have non-linear response characteristics.

The calibration of a printer involves the process of finding what set ofsignals must be sent to a printer to obtain a desired color. The desiredcolor is described in some device independent terminology (i.e. somewell defined standard), and the signals to the printer constitute adevice dependent terminology. A complete calibration will transform thedevice independent color description into a device dependent descriptionsuch that the resultant combination of materials (i.e. ink, toner, dye,etc.) on the paper produces the desired color (i.e. the color which wasinitially described in a device independent fashion).

The calibration of high quality printers can be divided into three majortasks, (1) setting gray balance (denoted by GB), (2) determining blackaddition (sometimes K+) and under color removal (termed UCR or sometimestermed "gray component replacement"); and finally (3) color correctionor color transformation.

Gray balance consists of determining what combination of inks are neededto produce a neutral gray of a desired density. Since black ink isneutral, only the combination of CMY inks must be determined. The blackchannel of the printer is linearized such that the reflectance fromblack ink is a linear function of the input signal. Because of unwantedabsorption, printer non-linearities and other effects, it is generallynot the case that equal amounts of CMY ink will produce a neutral color,but it is useful to define a CMY signal for which this is the case. Whenthe gray balanced printer is given equal amounts of CMY as inputsignals, it will put down the amounts of cyan, magenta, and yellow inkthat is needed to produce a neutral color. This generally means that theink amounts on paper will not be equal, but the final color will beneutral.

The procedure for gray balancing a printer involves printing manydifferent combinations of CMY ink, and then inspecting the resultingpatches to find the neutral colors. Then, the neutral patches are foundby measuring the color of the patches (there may be an initial visualsort of the patches). Hopefully, neutral patches of different densitiesare obtained, and then a curve fitting procedure is used to predict theink combinations needed for the other obtainable neutral grays. Thedifficulty with this procedure is the printing and inspection of manypatches, and that it requires the production, searching, and predictingto take place in a three dimensional space.

The process known as UCR involves determining how much CMY ink to removeafter a specified amount of black ink has been added to the same color.The idea here is to replace some of the CMY ink with black ink. Blackink is used to both extend the gamut of the printer, which aids in theproduction of sharper, cleaner looking images, and to reduce the totalamount of ink on the page.

The procedure for determining the amount of UCR is a strong function ofblack addition. An example of one such black addition strategy is thatof adding no black up until some minimum density, and then slowly addingmore black as the density of the requested color increases. In onepossible calibration strategy, the amount of black ink is zero untilsome minimum density, and then increases quadratically as a function ofrequested density.

One method of color calibration requires first performing a gray balancecalculation of the printer, followed by a determining K+ and UCR, andfinally making a determination of a color correction transform. The graybalance process requires searching through a three dimensional table,with interpolation to find a proper combination of inks to produce aneutral color. The second stage of the calibration requires the choiceof an initial K+ strategy and then the measurement of printed patches tomodify the K+ strategy and determine a UCR strategy. The final stage ofthe calibration requires the establishment of a color correctiontransformation. This is currently done by printing and measuring 1000(10×10×10) or 512 (8×8×8) patches distributed throughout the colorspace. These patches are used to build a three dimensional look-up-table(LUT) which is used with tetrahedral interpolation. In the longcalibration process, the table should only have to make minorcorrections to the colors; grays should already be gray (because of thegray balance calibration), and the colors should not changesignificantly in density (because the UCR calibration has been done).

In U.S. Pat. No. 4,500,919 to Schreiber, and U.S. Pat. No. 4,275,413 toSakamoto, the information derived from patch measuring was placed intolook-up tables, stored in a memory, perhaps ROM memory or RAM memorywhere the look-up table relates input color space to output color space.The look-up table is commonly a three dimensional table since colorspace is three dimensional. With a scanner or computer, the RGB spacecan be defined as three dimensional with black at the origin of a threedimensional coordinate system (0,0,0), and white at the maximum of athree dimensional coordinate system which an 8-bit system, would belocated at (255, 255, 255). Each of the three axes radiating from theorigin point therefore respectively define red, green, and blue. Asimilar construct can be made for the printer, with axes representingcyan, magenta, and yellow. Black is usually a separate toner which isadded separately. In the 8-bit system suggested there will be, however,over 16 million possible colors (256³) There are clearly too many valuesfor a 1:1 mapping of RGB to CMYK. Accordingly, as proposed in U.S. Pat.No. 4,275,413 to Sakamoto, only a relatively small number of samples aremade at the printer, perhaps on the order of 1,000, or even less.Therefore, the look-up tables consist of a set of values which could besaid to be the intersections for corners of a set of rectangularparallel-pipeds mounted on top of one another. Colors falling withineach rectangular volume can be interpolated from the measured values,through many methods including tri-linear interpolation, tetrahedralinterpolation, polynomial interpolation, linear interpolation, and anyother interpolation method depending on the accuracy of the desiredresult.

All of the references cited herein are incorporated by reference fortheir teachings.

SUMMARY OF THE INVENTION

In accordance with the invention, in a printer which produces images asa function of the combination of cyan (C), magenta (M), yellow (Y) andblack (K) colorants on an output print, there is provided a simplifiedcalibration process providing accurate printer response to ideal input.

In accordance with one aspect of the invention, in a printer whichproduces images as a function of the combination of cyan (C), magenta(M), yellow (Y) and black (K) colorants on an output print, responsiveto device independent colorimetric description of an image, there isprovided a method of calibrating the response of the printer to an imagedescribed in terms of colorimetric values, including the steps of first,determining a gray balanced or linear printer response; second, addingblack to an ideal device dependent description of a color image inaccordance with a predetermined black addition process, and third,providing a color correction transformation process, where the colorcorrection transformation is accomplished via a method of interpolatingprinter responses from a look up table indexing colorimetricdescriptions of measured real responses, including correction for graydensity and under color removal.

In accordance with another aspect of the invention, the table relatingdevice independent image descriptions to device dependent imagedescriptions is generated, with the entries to the table taking intoaccount the gray balanced or linearized response of the printer,desirable undercolor removal, and downstream black addition to thedevice dependent color description of the image.

In accordance with yet another aspect of the invention, there isprovided a method of calibrating a color printer so that color imagesdefined in terms of colorimetric color signals may be printed on a colorprinter responsive to printer colorant signals to render color printswith a set of three primary colorants and black on a substrate, thecalibration method comprising the ordered steps of:

a) gray balancing or linearizing the printer response;

b) determining, for a given density characteristic of a combination ofprimary colorant signals to generate a color on the substrate, a blackcolorant signal, to add black to a color print;

c) producing a color transform and correction table; and

d) storing an addressable mapping of colorimetric color signals tocolorant signals in a printer memory.

In accordance with still another aspect of the invention, there isprovided a method of printing in a calibrated color printer so thatscanned color images defined in terms of colorimetric color signals maybe printed on a color printer responsive to printer colorant signals torender a color print with a set of three primary colorants and black ona substrate, and including undercolor removal, the printing methodcomprising the ordered steps of:

scanning an image to derive a set of device independent colorimetriccolor signals;

converting said colorimetric color signals into device dependent primarycolorant signals, each primary colorant signal defining a density ofcolorant to be used in rendering a color print, said conversionaccounting for a subsequent black colorant addition;

determining, for a density of the combination of primary colorantsignals a black colorant signal, to add black colorant to the colorprint;

gray balancing or linearizing the primary colorant signals andlinearizing black to generate a set of corresponding printer colorantsignals to control the printer; and

using said printer colorant signals to control the printer to produce animage colorimetrically matching the scanned image.

These and other aspects of the invention will become apparent from thefollowing descriptions to illustrate a preferred embodiment of theinvention read in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of a scanning/printing system with colortransformation, for converting device independent image descriptions todevice dependent image descriptions;

FIG. 2 is a block diagram of a portion of the printing system of FIG. 1,showing modification of the device dependent image descriptions toaccommodate an individual printer; and

FIGS. 3A and 3B show the effect of the invention on color conversionlook-up tables.

Referring now to the drawings where the showings are for the purpose ofdescribing an embodiment of the invention and not for limiting same, abasic system for carrying out the present invention is shown in FIG. 1.In a simple system model, a scanner 10, such as perhaps the colorscanner available in the Xerox 5775 digital color copiers, which can becalibrated to produce a set of digital colorimetric or deviceindependent data describing a scanned image 12, which, by definition canbe defined in terms of r g b space. Resulting from the scanningoperation is a set of scanner image signals R_(s),G_(s),B_(s), definedin device dependent scanner terms. Incorporated into the scanner oranother processing path is a post-scanning processor 14, which providescorrection of scanner image signals R_(s),G_(s),B_(s) to colorimetricterms, typically digital in nature R_(c),G_(c),B_(c). The values may bein terms of CIE color space (rgb), or the L*a*b* luminance-chrominancespace (LC₁ C₂). A color space transform, indicated by block 20, such asthat described in U.S. Pat. No. 4,275,413 to Sakamoto, is used toconvert the device independent data to device dependent data. The outputof color space transform 20 is the image defined in terms of a devicedependent space, or colorant values C_(p), M_(p), Y_(p), K_(p) that willbe used to drive a printer 30. In one possible example, the colorantvalues represent the relative amounts of cyan, magenta and yellow tonersthat are to be deposited over a given area in an electrophotographicprinter, such as, again, Xerox 5775 digital color copiers. The printedoutput image may be said to be defined in terms of R_(p),G_(p),B_(p),which is hoped to have a relationship with R_(o),G_(o),B_(o) such thatthe printer has a color that is colorimetrically similar to the originalimage, although that similarity is ultimately dependent upon the gamutof the printing device.

With reference now to FIG. 2, and color space transformation and colorcorrection 20, initially, R_(c), G_(c), B_(c) color signals are directedto a three dimensional look up table stored in a device memory such as aROM or other addressable memory device, which will meet speed and memoryrequirements for a particular device. Color signals R_(c), G_(c), B_(c)are processed to generate address entries to the table which stores aset coefficients with which the R_(c), G_(c), B_(c) may be processed toconvert them to C_(x), M_(x), Y_(x) colorant signals. Values which arenot mapped may be determined through interpolation.

It will no doubt be recognized that there are many methods of providinga transform from device independent data to device dependent data, withU.S. Pat. No. 4,275,413 to Sakamoto describing one method, which itselfcan be varied. Once a conversion table is established, a method ofinterpolation referred to as tri-linear or cubic interpolation may alsobe used to calculate output values from the limited set of input values.The values stored in the look-up table can be empirically derived, as inSakamoto, or calculated or extrapolated based on empirical information,as in Po-Chieh Hung, "Tetrahedral Division Technique Applied toColorimetric Calibration for Imaging Media", Annual Meeting IS&T, NewJersey, May, 1992, pp. 419-422; Po-Chieh Hung, "Colorimetric Calibrationfor Scanners and Media", SPIE, Vol. 1448, Camera and Input ScannerSystem, (1991); and Sigfredo I. Nin, et al., "Printing CIELAB Images ona CMYK Printer Using Tri-Linear Interpolation", SPIE Proceedings, Vol.1670, 1992, pp. 316-324. The construction of the set of values storedfor the present invention will be discussed hereinbelow.

With reference again to FIG. 2, upon obtaining device dependent colorantsignals C_(x), M_(x), Y_(x), black addition (K+) is performed in twosteps. In the first step 50 the minimum density of the cyan, magenta,and yellow signals is determined. In the second step 60 it thengenerates a black colorant signal as a function thereof. As discussed,in one possible embodiment, the amount of black ink is zero until someminimum density, and then increases quadratically as a function ofrequested density. The addition of black ink is primarily an aestheticdetermination.

Subsequent to black addition, the color values are linearized, so thatlinearly increasing values of colorants produce a linearly increasingcolorimetric response. The linearization process is implemented via aset of look-up tables storing the responses of a set of patchesgenerated at a set of input values, where a curve fitting routine isused to map the set of possible input responses to characterized outputresponses. These values are then used to drive printer 30.

In an alternative embodiment of the invention, rather than linearizingthe color values, so that linearly increasing values of colorantsproduce a linearly increasing colorimetric response, the color valuesmay be gray balanced, so that equal amounts of color produce a neutralgray response at the printer. The gray balance process is implementedvia a set of look-up tables storing the responses of a set of patchesgenerated at a set of input values, where a curve fitting routine isused to map the set of possible input responses to characterized outputresponses. These values are then used to drive printer 30.

To create the color transformation table requires: 1) linearization orgray balance of printer responses; 2) determination of black additionstrategy; and 3) printing a set of patches with a selected set of inputvalues, taking into account linearization or gray balance and blackaddition. To create the table, a set of color patches are created, whichinclude determined linearization and black addition. This is done byprinting and measuring about 1000 (10×10×10) or 512 (8×8×8) patches ofprinter colors distributed throughout the color space, i.e., a large setof printer driving signals are generated, in varying densities ofcombinations of cyan, magenta and yellow, and used to drive the printer.The color of each patch is measured, using a spectrophotometer todetermine color in terms of R_(c) B_(c) G_(c). The measured colors ofthese patches are used to build a three dimensional look-up-table (LUT)relating R_(c) B_(c) G_(c) -defined colors to C_(x) M_(x) Y_(x) definedcolors. Conversions that do not include mapped and measured points maybe interpolated.

As was noted, the actual patch printed is a combination of cyan,magenta, yellow and black colorants, where black is added in accordancewith a black addition strategy. Additionally, the response of theprinter to signals for each color and black was linearized or graybalanced. Given a color patch created with 50% cyan, 50% magenta, 50%yellow (where the percentage of color refers to the percentage ofpossible density of that color), which should be approximately a neutralgray, perhaps 30% black will be added. Additionally, the signals will belinearized by look-up tables 70 and 80. When measured, however, thepatch color will be significantly darker than one might expect, due tothe black addition. Therefore, the R_(c) B_(c) G_(c) value mapped tothat combination of 50% cyan, 50% magenta, 50% yellow and 30% black hasa much higher density than would be expected. A corollary is that theamount of cyan, magenta, and yellow being used is less than would beexpected for the desired R_(c) B_(c) G_(c) density. Thus, it can be seenthat the amount of cyan, magenta, and yellow is lowered to compensatefor the amount of black added, and UCR is accomplished without a UCRdetermination step.

FIG. 3 provides a brief explanation. In FIG.3A, a 3-D look-up tableindexing R_(c) B_(c) G_(c) reflectance values to CMY reflectance valuesmay provide the relationship shown. Thus, a specified R_(c) B_(c) G_(c)signal produces a corresponding CMY signal. However, if black is added,the measured RGB for the same CMY is much darker. Accordingly, to obtainthe same RGB of FIG. 3A, in FIG. 3B, a lighter combination of CMY mustbe used. In other words, converting from device independent values todevice dependent values, in converting from RGB to CMY, the conversionmust provide for a lighter combination of CMY. The less densecombination of colorants accomplishes UCR.

The same measurements made and stored that account for UCR also accountfor gray balance or linearization determination, since the tablegeneration includes the response of the printer after compensation forgray balance or linearization.

The invention has been described with reference to a particularembodiment. Modifications and alterations will occur to others uponreading and understanding this specification. It is intended that allsuch modifications and alterations are included insofar as they comewithin the scope of the appended claims or equivalents thereof.

We claim:
 1. A method of printing in a calibrated color printer so thatscanned color images defined in terms of colorimetric color signals maybe printed on a color printer responsive to printer colorant signals torender a color print with a set of three primary colorants and black ona substrate, and including undercolor removal, the printing methodcomprising the ordered steps of:scanning an image to derive a set ofdevice independent colorimetric color signals; using a three dimensionallook up table stored in device memory, converting said colorimetriccolor signals into device dependent primary colorant signals, eachprimary colorant signal defining a density of colorant to be used inrendering a color print, said conversion derived with a calibrationprocess in which colorimetric color signals are mapped to primarycolorant signals using a calibration pattern including black, so thatvalues output from said three dimensional look up table reflect futureblack addition; determining, for a density of the combination of primarycolorant signals a black colorant signal, to add black colorant to thecolor print; linearizing each primary colorant signal and black togenerate a set of corresponding printer colorant signals to control theprinter; and using said printer colorant signals to control the printerto produce an image colorimetrically matching the scanned image.
 2. Amethod of printing in a calibrated color printer so that scanned colorimages defined in terms of colorimetric color signals may be printed ona color printer responsive to printer colorant signals to render a colorprint with a set of three primary colorants and black on a substrate,and including undercolor removal, the printing method comprising theordered steps of:scanning an image to derive a set of device independentcolorimetric color signals; using a three dimensional look up tablestored in device memory, converting said colorimetric color signals intodevice dependent primary colorant signals, each primary colorant signaldefining a density of colorant to be used in rendering a color print,said conversion accounting for a subsequent black colorant addition, andderived with a calibration process in which colorimetric color signalsare mapped to primary colorant signals using a calibration patternincluding black, so that values output from said three dimensional lookup table reflect future black addition; determining, for a density ofthe combination of primary colorant signals a black colorant signal, toadd black colorant to the color print; gray balancing the primarycolorant signals and linearizing black to generate a set ofcorresponding printer colorant signals to control the printer; and usingsaid printer colorant signals to control the printer to produce an imagecolorimetrically matching the scanned image.